It is common to use a single frequency synthesizer in an FM radio to generate both the transmit frequency source and the receive local oscillator ("LO") for down-conversion. This type of synthesizer is typical in applications where a device receives and transmits on the same channel at different points in time. A significant problem in this type of application is getting the synthesizer to switch between the transmit frequency and the receive LO frequency as quickly as possible. The time the synthesizer takes getting between these two frequencies is literally wasted by the radio since it can neither transmit or receive during this time period.
Some past approaches to reducing this switching time have included, for example, parameter optimization and adaptive filtering. Each of these approaches, however, has limitations. A problem with parameter optimization has been that it requires modifying the closed-loop bandwidth. A problem with adaptive filtering has been transients and loop response optimization.
As a result, there is a need for an improved fast-switching frequency synthesizer.